The first anisotropic matrix-bonded permanent magnets were made by the process of U.S. Pat. No. 2,999,275 (Blume). In that process, a dispersion of domain-size ferrite platelets in a nonmagnetic binder is milled or extruded to align the faces of the platelets mechanically. The highly-filled magnet of Example 1 of the patent has a B.sub.r of 2100 gauss and a maximum energy product of 0.9.times.10.sup.6 gauss-oersteds in the direction perpendicular of the faces of the aligned barium ferrite platelets.
U.S. Pat. No. 3,903,228 (Riedl) concerns a process for making improved barium ferrite platelets which by mechanical orientation provides a B.sub.r of 2690 gauss and a maximum energy product of 1.72.times.10.sup.6 gauss-oersteds (Example 4). Canadian Pat. No. 961,257 dated Jan. 21, 1975 teaches that by combining magnetic orientation with the mechanical orientation and using improved ferrite platelets, a B.sub.r of 2800 gauss and a maximum energy product of 1.89.times.10.sup.6 gauss-oersteds (Example 3) could be attained in a highly-filled magnet. The binder of Example 2 is a mixture of a thermoplastic, essentially amorphous, hot-melt polyamide resin having a softening point of 177.degree. C. and a sulfonamide plasticizer.
Instead of milling or extruding, highly-filled matrix-bonded ferrite magnets may be formed by injection molding while applying a magnetic field to align the ferrite particles as in U.S. Pat. No. 4,022,701 (Sawa). Barium ferrite magnets made by this process exhibit a B.sub.r up to 2528 gauss and a maximum energy product up to 1.57.times.10.sup.6 gauss-oersteds (Table 1), and for a strontium ferrite magnet a B.sub.r of 2680 gauss and a maximum energy product of 1.71.times.10.sup.6 gauss-oersteds.
In the process of U.S. Pat. No. 4,022,701, a spherical or equal-axes particle would be preferred since platelets rotating in response to a magnetic field tend to mechanically interfere with each other. Samarium-cobalt magnet powders such as those of Example 4 of U.S. Pat. No. 4,022,701 tend to have equal axes and thus are especially suitable for magnetic alignment.
From a study of the above-discussed and other prior art, the only report we can find of a highly-filled (i.e., at least 60 volume percent) matrix-bonded permanent magnet wherein the particles have an alignment exceeding 90% is the aforementioned Canadian Pat. No. 961,257 (which was granted to the company to which the present application is assigned). We believe that the process of the Canadian patent has never been commercialized and that it could not on a commercially practical basis be used to produce highly-filled matrix-bonded magnets having a degree of particle alignment consistently exceeding 90%. The following formula gives an approximation of the degree of particle alignment in a matrix-bonded magnet: EQU B.sub.r /(4.pi..sigma.d)V
where .sigma. is the magnetic moment of the particles, d is the density of the particles and V is the volume percent of the particles in the matrix-bonded magnet.